1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine and a printer.
2. Description of the Related Art
Conventionally, a color image forming apparatus capable of outputting a full-color image in the following manner has been put into practical use. That is, toner images formed on surfaces of respective image bearing members are sequentially and temporarily transferred onto a surface of an intermediate transfer member for each color (hereinafter referred to as “primary transfer”) so that a full-color superimposed image is formed on the surface of the intermediate transfer member. After that, the full-color toner image on the surface of the intermediate transfer member is transferred onto a recording material (hereinafter referred to as “secondary transfer”).
In such an image forming apparatus, when there is a speed difference between the moving speed of the surface of the intermediate transfer member and the moving speed of the surface of the image bearing member, in a primary transfer step of superimposing the toner images of the plurality of colors from the image bearing members onto the intermediate transfer member, a phenomenon that images of the respective colors are misaligned may occur in some cases. In particular, a significant misregistration tends to occur between the toner image of a color that is primarily transferred onto the surface of the intermediate transfer member at the beginning and the toner image of a color that is primarily transferred onto the surface of the intermediate transfer member at the end. This phenomenon occurs because the amount of toner that exists at a contacting portion between the image bearing member and the intermediate transfer member (hereinafter referred to as “primary transfer portion”) differs when the toner image of each color is transferred. In a primary transfer step of the first color, the primary transfer is performed on the surface of the intermediate transfer member under a state in which substantially no toner is provided on the surface, whereas in primary transfer steps of the second and subsequent colors, the primary transfer is performed on the surface of the intermediate transfer member under a state in which toner is already provided. Therefore, the frictional force acting between the image bearing member and the intermediate transfer member at the primary transfer portion differs depending on the presence and absence of toner at the primary transfer portion. Therefore, the moving speed of the surface of the intermediate transfer member changes, and thus misregistration of toner images occurs among the colors.
Japanese Patent Application Laid-Open No. 2012-42754 discloses the following configuration. At the primary transfer portion between the intermediate transfer member and the image bearing member arranged on the uppermost stream, waste toner is supplied as a lubricant. In this manner, fluctuations in circumferential surface speed, which are generated in the intermediate transfer member during an image forming operation, can be reduced, and thus occurrence of color misregistration can be reduced.
However, the waste toner remaining on the intermediate transfer member after a secondary transfer step may contain paper powder generated from the recording material in some cases. The waste toner containing paper powder has low flowability because foreign matters are mixed therein. The waste toner having low flowability is unstable in lubricity. Therefore, when the waste toner is used as a lubricant, it is difficult to reduce the fluctuations in circumferential surface speed, which are generated in the intermediate transfer member during the image forming operation.